This module hooks '$SIG{__WARN__}' and converts warnings to the Test::More
manpage's 'fail()' calls. It is designed to be used with 'done_testing',
when you don't need to know the test count in advance.
Just as with the Test::NoWarnings manpage, this does not catch warnings if
other things localize '$SIG{__WARN__}', as this is designed to catch
_unhandled_ warnings.